Most infanticides occur when infants' mothers are not accompanied by their group's mature male (usually because he has died). Infants in this situation are almost certain to be killed by unfamiliar males unless they are nearly weaned. Active defense of infants by females is ineffective, and females cannot avoid unfamiliar males for prolonged periods. In contrast, infanticide is rare — yet has been observed — in encounters between mature males. It is not associated with group takeovers and male eviction by extra-group males, unlike the case in many other mammals. Demographic constraints and reproductive competition limit the occurrence of defensive coalitions between males. These factors, plus the high risks associated with male/male aggression, inhibit the occurrence of group takeovers by male coalitions. Infanticide shortens interbirth intervals and results in a high probability that a female will mate with the infanticidal male.